Integrated circuit (semiconductor) memory devices can be roughly categorized into volatile memory devices and nonvolatile memory devices according to whether the semiconductor memory devices maintain data when a power supply is cut off. The volatile memory devices may include, for example, DRAM devices and SRAM devices and the nonvolatile memory devices may include, for example, flash memory devices. These conventional memory devices may indicate a logic value “0” or a logic value “1” according to the relative presence or absence of stored charges.
Recently, phase changeable memory devices have been proposed. A phase changeable memory device uses phase changeable materials that change their crystalline states depending on a resistive heating by a current pulse. Conventionally, chalcogenide compounds (GST) including germanium (Ge), antimony (Sb) and/or tellurium (Te) may be used as a phase changeable material.
An amorphous phase changeable material has relatively high electric resistance, and the electric resistance decreases as the amorphous phase changeable material changes gradually into a crystalline state. Since the resistance can vary according to the crystalline states, logic information can be determined by sensing a difference between the resistances. The phase changeable material layer can contact a conductive plug that passes through an insulating layer, and may be provided with heat (Joule heat) from the conductive plug and/or a heater plug. The conductive plug is conventionally formed of conductive nitride materials such as titanium nitride. To embody a phase changeable memory device having low power consumption, the diameter of the conductive plug may be reduced.
However, if the conductive plug is formed of titanium nitride, it may be difficult to reduce the diameter as much as may be desirable. In particular, a heater plug having about 40 nm diameter may not endure a current density of about 108 A/cm2 or more under a write current of about 1 mA, because the titanium nitride may have weak tolerance with respect to this current density. As a result, if a conventional titanium nitride heater plug is used, the diameter of the heater plug may not be reducible under about 40 nm. It therefore may be difficult to provide phase changeable memory devices having a desirably high integration density and/or a desirably low power consumption.